The present invention relates to a ratio control system for a continuously variable transmission for example for use in automotive vehicles.
Ratio control systems for continuously variable transmissions are well known in the art. Such a ratio control system is disclosed in co-pending U.S. patent application Ser. No. 411,987, filed Aug. 26, 1982 (now U.S. Pat. No. 4,515,040) which corresponds to European Patent Application No. 82 107 823.5. In this ratio control system, a target or desired reduction ratio is determined based on various input signals representative of a variation of the vehicle's operating condition, and a pulley unit, including a drive pulley, a driven pulley and a V-belt, is hydraulically controlled such that an actual reduction ratio is varied in the proper direction to decrease the deviation from the target reduction ratio.
Generally, continuously variable transmissions include a drive pulley, a driven pulley, a V-belt drivingly interconnecting such pulleys, a source of hydraulic fluid pressure, a pressure regulator valve for regulating the hydraulic fluid pressure to develop a control pressure variable with actual reduction ratio and engine's load (induction manifold), a hydraulic a fluid network including a shift control valve establishing fluid interconnection among the regulator valve, drive pulley, and driven pulley for controlling fluid pressure supplied to the drive pulley relative to that supplied to the driven pulley so as to vary the reduction ratio in response to the position of an adjustable control member. The position of the adjustable control member is controlled by a microcomputer based control unit wherein the reduction ratio is varied such that the deviation of an actual reduction ratio from a target reduction ratio is decreased. A ratio control system of this kind is disclosed in co-pending U.S. patent application Ser. No. 543,838, filed Oct. 20, 1983, now U.S. Pat. No. 4,576,265 (corresponding to EP No. 83 110 546.5).
In a ratio control system of the above kind, the deviation from the target reduction ratio is integrated and then mulitplied with a predetermined constant (integral gain) to provide an integral control factor, and the deviation is multiplied with another predetermined constant (proportional gain) to provide a proportional control factor. The integral and proportional control factors are added to each other and based on this result, a control unit generates a shift command signal to a shift actuator, such as a stepper motor, for adjusting the position of an adjustable control member. The integral and proportional control gains are set to constants, respectively. Since shifting response is proportional to the line pressure such that it is slow when the line pressure is low, while it is fast when the line pressure is high, the ratio control system cannot provide shifting response to satisfy drivability and stability criteria over varying level of the line pressure if the integral and proportional control gains are kept unchanged with variations of the line pressure. In other words, if the control gains are set to optimum constants in view of a certain level of the line pressure, the control system provides slower shifting response than required when the line pressure is lower than this level or faster shifting response than required when the line pressure is higher than this level. Therefore, the conventional ratio control system fails to accomplish a desired operating condition, for example, operating condition where the fuel consumption is minimum, over varying level of line pressure.